docs: add contributor guide for adding a new page route (closes #426)

[LaPoshBaby]

Summary

• Adds docs/adding-page-routes.md: a contributor guide covering where routes are registered in src/App.tsx, the file naming convention for page components (PascalCase + Page suffix, default export, in src/pages/), the recommended pattern for auth-protected routes using a RequireAuth wrapper around useAccount().isConnected from wagmi, and a four-step worked example adding a public AboutPage at /about.
• Adds a one-line pointer in CONTRIBUTING.md under "Frontend conventions" so contributors can find the guide.

Testing

• pnpm lint passed (only one unrelated pre-existing warning in CreatorCard.tsx:119).
• pnpm build not run; this is a docs-only change and no source code was modified.

Checklist

• Linked issue or backlog item — Closes Add docs for how to add a new page route to the client #426
• Scope is limited to the stated change
• Updated docs if behavior or setup changed — this PR is the docs change
• Added screenshots for UI changes when relevant — N/A, no UI changes

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